A compact disc (CD) enables the recording of 74 minutes of music data or 650 MB of digital data using an optical system including a light source having a wavelength of 780 nm and an objective lens having a numerical aperture of 0.45. Further, a digital versatile disc (DVD) enables the recording of 2 hours and 15 minutes of MPEG2 moving pictures and 4.7 GB of digital data using an optical system including a light source having a wavelength of 650 nm and an objective lens having a numerical aperture of 0.6.
In recent years, reflecting the broadcasting of high-definition moving pictures having a horizontal resolution of 1000 pixels and the high functionalization of personal computers, there have been even more expectations for high-density and high-capacity optical discs. In response to such expectations, an optical disc system including a combination of a light source having a wavelength of about 400 nm and an objective lens having a numerical aperture of 0.85, and the like have been proposed and a recording capacity exceeding 20 GB per side is about to be realized.
In this way, optical disc devices have realized higher data recording densities on discs using light sources having shorter wavelengths and objective lenses having larger numerical apertures. However, such an approach to high density recording by the shorter wavelength of the light source and the larger numerical aperture of the lens is coming closer to a limit. Specifically, in a wavelength range equal to or below 400 nm, it is difficult to control aberrations since the wavelength dispersion of a glass material used for the lens becomes large. Further, the use of a solid immersion lens technology being developed to increase the numerical aperture causes problems such as difficulty in exchanging discs since a lens working distance, i.e. a distance between the lens and the disc becomes extremely short (about 50 nm). Thus, a great deal of attention is being drawn to the holographic recording technology in order to overcome these problems and realize high data recording density on discs.
FIG. 20 is a diagram of an optical disc system employing shift multiplexing recording and, for example, proposed by Psaltis et al, showing the schematic construction of an optical system of this optical disc system (see, for example, patent literature 1). An optical disc system 200 employing shift multiplexing recording and shown in FIG. 20 includes a laser light source 201, a beam expander 207, a polarizing beam splitter 208, a mirror 210, a spatial light modulating element 202, Fourier transform lenses 203 and 204, a holographic disc 205, a half wavelength plate 211, a condenser lens 212 and a two-dimensional light receiving element array 206.
A light from the laser light source 201 is split in the polarizing beam splitter 208 after having the diameter thereof expanded by the beam expander 207. One split beam has a propagating direction thereof changed by the mirror 210 and transmits through the spatial light modulating element 202 after transmitting through the half wavelength plate 211. The beam having transmitted through the spatial light modulating element 202 is condensed by the Fourier transform lens 203, and the condensed beam is irradiated onto the holographic disc 205 as a signal light 220. The other split beam is condensed by the condenser lens 212, and this condensed other beam becomes a reference light 222 and is irradiated onto the same position on the holographic disc 205 as the irradiated position of the signal light 220. The holographic disc 205 is constructed by sealing a holographic medium such as a photopolymer between two glass substrates, and interference fringes of the signal light 220 and the reference light 222 are recorded on the holographic medium.
Here, the spatial light modulating element 202 includes an optical switch array in which optical switches are two-dimensionally arrayed and independently turned on and off in response to an input signal 223. Each optical switch is a cell corresponding to 1 bit of image information. For example, the spatial light modulating element 202 having 1024 cells×1024 cells can simultaneously display 1 MB of information. The 1 MB of information to be displayed on the spatial light modulating element 202 is converted into a two-dimensional beam array upon transmitting through the spatial light modulating element 202 and condensed by the Fourier transform lens 203.
Upon reproducing a recorded signal, only the reference light 222 is irradiated onto the holographic disc 205. Then, a reproduced signal light 221, which is a diffracted light from the holographic disc 205, is converted into a two-dimensional beam array upon transmitting through the Fourier transform lens 204, and this beam array is irradiated onto the two-dimensional light receiving element array 206.
Here, the two-dimensional light receiving element array 206 is such that light receiving elements are two-dimensionally arrayed and corresponds to the optical switch array in the spatial light modulating element 202. Accordingly, the respective beams in the two-dimensional light beam array are photoelectrically converted by the corresponding light receiving elements in the two-dimensional light receiving element array 206 and a reproduced signal 224 is outputted.
The characteristic feature of the optical disc system 200 shown in FIG. 20 is to enable angle multiplexing recording, i.e. the multiplexing recording of information by changing incident angles of the signal light and the reference light on the optical disc since the holographic medium is as thick as about 1 mm and the interference fringes are recorded as thick gratings, i.e. Bragg gratings, whereby it can be realized to enlarge the capacity of information to be recorded in the optical disc. In the optical disc system shown in FIG. 20, angle multiplexing is reached by shifting the irradiated position of the reference light having a spherical wave instead of changing the incident angle of the reference light 222. This system exploits slight changes of the incident angle of the reference light sensed by the respective parts of the holographic disc 205 when the holographic disc 205 is slightly rotated to shift the recording position.
In such a construction, a large volume of data can be recorded in one hologram and parallel recording and reproduction are possible. Thus, a high-speed optical information recording/reproducing device can be realized. Further, since multiplexing recording is possible, a large-capacity optical information recording/reproducing device can be realized.
However, since the light amount of the reproduced diffracted light is small in the above conventional construction, electric noise created in the two-dimensional light receiving element array 206 becomes relatively large, thereby reducing the S/N ratio of the reproduced signal. In order to reproduce the signal with a sufficient S/N ratio, the transfer rate had to be decreased. Further, recording could be made in the same kinds of media only at the same recording density in the above conventional construction.
Further, since the signal light is cut off in the respective cells of the spatial light modulating element 202 in the above conventional construction, light utilization efficiency is low. Thus, a coherent light source having a large output was needed. Furthermore, the transfer rate during the recording was low. Further, in the above conventional construction, the magnification of the lens changes with time or with ambient temperature, which has caused a problem that a spot array of the reproduced signal light 221 on the two-dimensional light receiving element array 206 do not coincide with the light receiving cells of the two-dimensional light receiving element array 206 to deteriorate the reproduced signal.
Further, in the above conventional construction, the positions of the spots of the reproduced signal light 221 do not coincide with the positions of the light receiving cells of the two-dimensional light receiving element array 206 due to the distortion of the lens at the time of reproducing the signal, thereby causing a problem of deteriorating the reproduced signal. Furthermore, in the above conventional construction, the lens needs to be designed to strictly satisfy an fsin θ condition so that the distortion of the lens is equal to or below 0.2%, thereby causing problems of increasing the number of constituent lens of the lens, the weight of the lens and the cost of the lens.
The above conventional construction also had a problem of taking a long time to verify the information recorded in the holographic disc 205. This is because the rotating speed of the holographic disc 205 is very slow, and the reproduction of the recorded holograms for verification took a time for at least one turn of the disc. For example, if the holograms are continuously recorded in the circumferential direction of the holographic disc at intervals of 30 micrometers at a radius position of 40 mm of the holographic disc 205, the linear velocity of the disc is 30 mm/second and the circumferential distance when the disc makes one turn is about 250 mm. Since the number of revolutions of the holographic disc is constant during the recording, it takes about 8 seconds from the recording to the verification.
Further, the above conventional construction had a problem of taking a long time to fix the recording material of unrecorded parts when the recording is finished. Further, since the pitch of the light receiving cells of the two-dimensional light receiving element array 206 is substantially equal to that of the spot array of the reproduced signal light 221 in the above conventional construction, there was a likelihood that the pitch of the light receiving cells of the part or entirety of two-dimensional light receiving element array 206 became larger than that of the spot array of the reproduced signal light 221 due to the change of the magnification of the lens, the distortion of the lens and the like with ambient temperature and/or time, thereby considerably deteriorating the reproduced signal.
In another conventional construction, the pitch of light receiving cells of a two-dimensional light receiving element array is at least half as large as that of a spot array of a reproduced signal light. Since the number of the cells of the two-dimensional light receiving element array increases, there was problems of increasing the cost of the two-dimensional light receiving element array and signal lead lines from the two-dimensional light receiving element array and enlarging the circuit scale of a two-dimensional data processing circuit.
Further, since the size of the light receiving cells of the two-dimensional light receiving element array 206 is substantially equal to the pitch of the light receiving cells of the two-dimensional light receiving element array 206 in the above conventional construction, there was a problem that the influence of intersymbol interference from adjacent spots of the reproduced signal light 221 became large to deteriorate the reproduced signal. Furthermore, since the size of the cells of the spatial light modulating element 202 is large in the above conventional construction, the size of the spots of the reproduced signal light 221 becomes large. Thus, there was a problem that the influence of intersymbol interference became large to deteriorate the reproduced signal.
Further, since the first null positions of the spot of the reproduced signal light 221 differ from the positions of the adjacent cells of the two-dimensional light receiving element array 206 in the above conventional construction, there was a problem that the influence of intersymbol interference became large to deteriorate the reproduced signal. Further, in the above conventional construction, there are provided apertures having the size of a 0th-order light of a diffraction pattern determined by the apertures of the cells of the spatial light modulating element 202 when the signal light 220 is incident on the holographic disc 205. Thus, there was a problem that the spot size of the signal light 220 on the two-dimensional light receiving element array 206 became large to reduce the quality of the reproduced signal by the intersymbol interference.
PATENT LITERATURE 1: U.S. Pat. No. 5,671,073